Cryogenic distillation of air is the predominant source for the production of oxygen (O2) and nitrogen (N2). Prior to distillation, ambient air must be purified to remove or reduce high boiling contaminants. For example, high boiling contaminants such as H2O and CO2 must be removed to certain levels (e.g., the ppm level or below) in order to ensure continuous operation of the distillation unit. Failure to remove high boiling contaminants to acceptable levels prior to the air being fed to the distillation column(s) can result in freezeout in the cryogenic equipment and/or concentration of hydrocarbons and/or N2O in oxygen-rich streams.
Air prepurification for cryogenic distillation of air typically employs one of two general classes of adsorption systems: temperature swing adsorption (TSA) systems and pressure swing adsorption (PSA) systems. For both PSA and TSA prepurification systems, the adsorption cycle usually has at least two stages of operation. In one stage, contaminants (such as H2O, CO2, C2H2, N2O) are adsorbed. In another stage, the contaminant laden bed is regenerated/purged of the adsorbed contaminants. In TSA systems, adsorption will typically occur near ambient temperature (e.g., 40-50° F.) and regeneration will usually take place at high temperature (e.g., 400-500° F.).
In PSA-based prepurification systems, contaminant removal (such as H2O, CO2, C2H2, N2O) usually takes place at relatively constant temperature (e.g., 50-90° F.). The adsorbent is regenerated by pressure reduction and purge with clean waste gas.
When considering air prepurification alone (i.e., excluding considerations concerning the distillation process), there is a natural motivation to increase operating pressure. Increased air pressure enables a substantial reduction in water content through simple cooling and direct phase separation. Consequently, the quantities of adsorbent and vessel volume can be reduced. This can translate into economical benefits associated with equipment construction and transportation.
In systems employing PSA prepurification, there often exists the need to provide minor/auxiliary streams of air to the air distillation process. Such streams can be effectively employed for the reduction of power consumption during times of high power cost. Alternatively, such streams can be used to adjust product mix or pressure. Unfortunately, extraction of such streams from PSA prepurification systems designed for the highest common air pressure have not provided such streams without undue added complexity and cost.
U.S. Pat. No. 4,964,901 to Rhode relates to a process in which two air streams are prepurified and directed to a cryogenic air separation process. The process employs a high and low pressure column. The process further relates to low purity O2 generation and the use of lower pressure purification for generating a feed that is directed to the lower pressure column.
U.S. Pat. No. 5,661,987 to Zarate discloses a three bed PSA unit for use in conjunction with a cryogenic air separation process. The process shown depicts the PSA unit after the main air compressor. The entire prepurified air stream is then directed to the cryogenic distillation unit.
U.S. Pat. No. 5,571,309 to Kumar relates to the use of a prepurification system for application with a cryogenic air separation unit (ASU) in which the PSA system serves to prepurify two air streams of different pressure.
U.S. Pat. No. 6,536,234 B1 to Shah depicts a three column (pressure) system with two air feed streams of differing pressure.
In the context of PSA air prepurification systems for cryogenic air separation, it would be desirable to provide a PSA prepurification system configured to operate at a pressure near or below that which exists in the highest pressure column or the highest common air pressure of the cryogenic separation unit. It would further be desirable to provide an integrated process which seeks to reduce overall cost associated with respect to air separation processes employing PSA prepurification.